1. Field of the Invention
This invention relates to a digital angle encoder and more particularly to a tracking digital angle encoder which utilizes a resolver, a translator, a threshold detector and a counter.
2. Description of the Prior Art
Prior art digital angle encoders utilize multiple gear discs in an arrangement where each disc is read by a device such as a light emitting diode and a phototransistor arrangement. Experience has shown in many industrial applications that vibration and handling of prior art encoders has caused substantial maintenance and down time. Prior art digital angle encoders also are capable of only providing a discrete indication of the shaft position. That is, they cannot indicate the shaft position continuously over its entire rotation with essentially infinite resolution.
An absolute digital position encoder cannot provide a continuous indication of the shaft position. That is, the prior art digital encoders must necessarily break the shaft position down into a discrete number of intervals or angles. Once selected, the size of these intervals or angles cannot be adjusted, and also, the movement or position of the shaft within each discrete interval is indeterminable. Since the interval or angle cannot be adjusted, the digital counts corresponding to a given rotation are fixed. Whenever the term resolver is utilized herein, it is understood to mean resolver, synchro, differential transformer, control transformer or any other sinusoidal position indicating device. The output of resolvers are normally in suppressed carrier form, but it is customary to discuss the output as representing the cosine and sine of selected analog angles. Whenever sine and cosine functions are discussed herein, it is to be understood that these can represent signals in absolute or suppressed carrier form.
U.S. Pat. No. 3,609,320 describes a digital measuring system whereby the position of a movable member is measured utilizing a multiple counter technique which develops sine and cosine signals in Pulse Width Modulated form for application to a trigonometric type angle transducer. An error signal is generated by the transducer to control various logic subsystems that direct the counting of certain counters. The system uses a position measuring device having a plurality of operating cycles for generating an error signal as a function of the position of the movable member relative to the workpiece.
U.S. Pat. No. 3,686,487 also describes a digital measuring system that employs trigonometric signal generators. The system includes a digital to analog converter method to generate two or more analog output signals as a function of a digital input. The analog outputs which are Pulse Width Modulated rectangular waveforms include a fundamental sinusoidal frequency component having an amplitude proportional to a trigonometric function of the digital input. The analog outputs are typically connected as inputs to a position measuring device to trigonometrically define the position between two members of the position measuring transducer.
Like U.S. Pat. No. 3,609,320, the system of the 3,686,487 patent employs the position measuring device as an angle receiver, accepting Pulse Width Modulated signals derived from the array of electronic circuits.